The University of Michigan researchers said the urge to overeat tasty treats comes from an unexpected area of the brain called the neostriatum, which produces an opium-like chemical that enhances such desire and may be partly responsible for overeating among people.

"Previously, people thought this area of the brain was only involved in motor function and learning, but we found it's involved in motivation and generating instant consumption," said lead researcher Alexandra DiFeliceantonio, a graduate student in biopsychology at the university.

This finding might have implications for people, she added, noting that it may be possible in the future to target the area with a drug that could block the impulse to overeat and thus may help people lose weight.

Experts note, however, that results in animal studies often don't translate to humans.

The new report was published Sept. 20 in the journal Current Biology.

For the study, DiFeliceantonio and her colleagues gave lab rats a drug to artificially boost the action of the neostriatum. The animals were then given M&Ms, and proceeded to eat twice as many as they normally would, she said.

"That's the same as a 150-pound human eating seven pounds of M&Ms in an hour," DiFeliceantonio said.

In addition, the researchers noted that the amount of a chemical called enkephalin, produced in the neostriatum, increased when the animals ate the chocolate treats.

It is the increased production of this chemical that increases the desire to overeat sweet and fatty foods, DiFeliceantonio said. When presented with a choice of usual food or M&Ms, the rats with high levels of the chemical consistently ignored their regular food and gorged on chocolates.

Preferring sweet and fatty foods is probably one of the things that helped humans to thrive, said Dr. David Katz, director of the Yale University Prevention Research Center in New Haven, Conn.

"We tend to like flavors, such as sweet, that in nature are associated with life-sustaining foods, and tend to dislike flavors, such as bitter, more often associated with toxins," he said.

The impulses that once helped keep our ancestors from starving, however, now may contribute to eating disorders and epidemic obesity, Katz added.

"But the fault here is not with the world within us, which is the same as it ever was," he said. It is with what and how much people eat, which has made the brain's natural functioning "backfire rather badly."

Another expert thinks the research could have implications for addiction and obesity.

Paul Sanberg, director of the Center of Excellence for Aging and Brain Repair at the University of South Florida College of Medicine in Tampa, said the findings "suggest areas of the human brain that might be looked at for eating disorders or addiction."